<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
  <link rel="stylesheet" media="screen" type="text/css" href="./style.css" />
  <link rel="stylesheet" media="screen" type="text/css" href="./design.css" />
  <link rel="stylesheet" media="print" type="text/css" href="./print.css" />

  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
</head>
<body>
<div class="dokuwiki export">

<h1 class="sectionedit1"><a name="data_structure_design_discussion" id="data_structure_design_discussion">Data structure design discussion</a></h1>
<div class="level1">

</div>
<!-- EDIT1 SECTION "Data structure design discussion" [1-48] -->
<h1 class="sectionedit2"><a name="concept_diagram" id="concept_diagram">Concept diagram</a></h1>
<div class="level1">

<p>
(Inspired by gnetman, by Bill Cox)
</p>

<p>
<a href="/./lib/exe/fetch.php?hash=d4505f&media=http%3A%2F%2Fwww2.eng.cam.ac.uk%2F~pcjc2%2Fgeda%2Fdatastructures.png" class="media" title="http://www2.eng.cam.ac.uk/~pcjc2/geda/datastructures.png"><img src="exe/fetch.php?hash=d4505f&media=http%3A%2F%2Fwww2.eng.cam.ac.uk%2F~pcjc2%2Fgeda%2Fdatastructures.png" class="media" alt="" /></a>
</p>

</div>
<!-- EDIT2 SECTION "Concept diagram" [49-178] -->
<h2 class="sectionedit3"><a name="concepts_behind_the_structures" id="concepts_behind_the_structures">Concepts behind the structures</a></h2>
<div class="level2">

</div>
<!-- EDIT3 SECTION "Concepts behind the structures" [179-222] -->
<h3 class="sectionedit4"><a name="design" id="design">Design</a></h3>
<div class="level3">

<p>
This is might not exist as a “file”, as such, but exists as a data structure entity to be the owner of the circuits required in a particular design. The “root circuit” is the uppermost level of hierarchy.
</p>

</div>
<!-- EDIT4 SECTION "Design" [223-446] -->
<h3 class="sectionedit5"><a name="circuit" id="circuit">Circuit</a></h3>
<div class="level3">

<p>
A <strong>circuit</strong> entity is the key concept in this model. It defines an electrical block by a its external connections (<strong>MPort</strong>s). A schematic is one way of representing a circuit, hence a circuit object may own or more <strong>page</strong> of schematics.
</p>

<p>
We may also define a <strong>symbolic</strong> (graphic) representation of a circuit - this is like a schematic <strong>page</strong>, however its representation should fit within a single sheet. The minimum a symbolic representation must contain is the <strong>pins</strong> which connect it to higher levels of circuit hierarchy.
</p>

</div>
<!-- EDIT5 SECTION "Circuit" [447-1004] -->
<h3 class="sectionedit6"><a name="mport" id="mport">MPort</a></h3>
<div class="level3">

<p>
If it is to be useful as a re-usable block, a sub-<strong>circuit</strong> must expose electrical connectivity for a parent <strong>circuit</strong> to connect with. Each such connection is represented by an <strong>Mport</strong> (Master port). This term (re-used from gnetman) represents the fact that once a circuit is instantiated, we need to differentiate between the connections of each specific instance. This is done with instance specific <strong>Port</strong> structures. The <strong>port</strong>s point back at the <strong>Mport</strong>s (master ports) of the circuit representation.
</p>

</div>
<!-- EDIT6 SECTION "MPort" [1005-1541] -->
<h3 class="sectionedit7"><a name="instance" id="instance">Instance</a></h3>
<div class="level3">

<p>
A <strong>circuit</strong> represents a re-usable electrical entity which we may replicate at various points in our design hierarchy. This is done by instantiating the sub-<strong>circuit </strong> in a higher level of hierarchy. Each instance is associated with an <strong>Instance</strong> structure, which is a placeholder for instance specific attributes such as the sub-circuit&#039;s hierarchical refdes.
</p>

</div>
<!-- EDIT7 SECTION "Instance" [1542-1929] -->
<h3 class="sectionedit8"><a name="attrib" id="attrib">Attrib</a></h3>
<div class="level3">

<p>
An <strong>Attrib</strong> defines meta-data attached which might be attached to a <strong>circuit</strong>, a <strong>circuit</strong>&#039;s <strong>Mport</strong>, a specific <strong>circuit</strong> <strong>instance</strong>, or a <strong>Net</strong>.
</p>

<p>
In a break from gEDA&#039;s current <strong>attrib</strong> model, it makes sense to associate the meta-data directly with the particular entity it pertains to, rather than the graphic representation. This is because some forms of sub-<strong>circuit</strong> entity may be defined without a schematic, and could still require this meta-data. It will be possible to reference any <strong>attrib</strong> within the realm of a <strong>circuit</strong> for display on its schematic <strong>page</strong>(s) where that is desired.
</p>

</div>
<!-- EDIT8 SECTION "Attrib" [1930-2569] -->
<h3 class="sectionedit9"><a name="netlist" id="netlist">Netlist</a></h3>
<div class="level3">

<p>
A <strong>Netlist</strong> defines the electrical connectivity of a <strong>circuit</strong>. It owns a number of <strong>Net</strong>s, which individually represent a single connection between <strong>Mport</strong>s belonging to this <strong>circuit</strong>, and <strong>ports</strong> of instantiated sub-<strong>circuits</strong>. 
</p>

<p>
Initially, it is likely there will only be one netlist for a <strong>circuit</strong> - the one constructed from processing the electrically relevant objects on <strong>page</strong>(s) of the <strong>circuit</strong>&#039;s schematic.
</p>

<p>
Future developments may see multiple netlists for a circuit, possibly some generated / written in an HDL language, and critically, re-exported from a layout package (e.g. PCB). It will be possible to identify and flag up differences in connectivity throughout a design flow, be that from HDL to schematic, or schematic to layout.
</p>

<p>
This has real applications in back-annotation and in design verification.
</p>

</div>
<!-- EDIT9 SECTION "Netlist" [2570-3435] -->
<h3 class="sectionedit10"><a name="net" id="net">Net</a></h3>
<div class="level3">

<p>
A <strong>net</strong> associates with structures forming a given electrical connection within this <strong>circuit</strong>.
</p>

<p>
As we also have a graphical representation of the wires (<strong>ConnSegment</strong>s) which make up this connection, each <strong>Net</strong> can be associated with multiple <strong>ConnSegment</strong>s. The association to <strong>Pins</strong> representing <strong>Mport</strong>s of this <strong>circuit</strong> and to the <strong>Pins</strong> of any instantiated sub-<strong>circuits</strong> is made via a <strong>net</strong>&#039;s association to the appropriate <strong>Mport</strong> and <strong>port</strong> structures.
</p>

</div>
<!-- EDIT10 SECTION "Net" [3436-3940] -->
<h3 class="sectionedit11"><a name="page" id="page">Page</a></h3>
<div class="level3">

<p>
A <strong>page</strong> is a canvas for placing graphical objects representing a circuit. A <strong>page</strong> can be used to draw an electrically meaningful schematic, or it can be used to draw a symbolic representation of the circuit entity.
</p>

<p>
Whilst most objects on a <strong>page</strong> are graphic primitives, there are some which have a relation to the <strong>circuit</strong>&#039;s electrical specification.
</p>
<ul>
<li class="level1"><div class="li"> <strong>ConnSegments</strong> (or <strong>net</strong>s) represent connected electrical signals within the circuit represented.</div>
<ul>
<li class="level2"><div class="li"> A connectivity representation (<strong>netlist</strong>) can be built by considering the end-point positioning of these objects.</div>
</li>
<li class="level2"><div class="li"> <strong>ConnSegment</strong> is intended to be a generalisation of <strong>net</strong>s and <strong>bus</strong>es for the purpose of this diagram.</div>
</li>
</ul>
</li>
</ul>
<ul>
<li class="level1"><div class="li"> <strong>Pins</strong> represent a connection outside this circuit.</div>
<ul>
<li class="level2"><div class="li"> When constructing a netlist, coincidence of a <strong>ConnSegment</strong> end on these implies an electrical connection to that external port.</div>
</li>
<li class="level2"><div class="li"> Each <strong>pin</strong> (or group of pins?) represent an external electrical connection with this <strong>circuit</strong>.</div>
</li>
<li class="level2"><div class="li"> There is a necessary link between a <strong>pin</strong> and the circuit&#039;s <strong>Mport</strong> which it represents.</div>
</li>
</ul>
</li>
</ul>
<ul>
<li class="level1"><div class="li"> <strong>complex</strong> objects represent instantiating a sub-<strong>circuit</strong>, and will be linked to a specific <strong>instance</strong> structure.</div>
<ul>
<li class="level2"><div class="li"> Graphically, this means a <strong>symbolic</strong> representation of the instantiated circuit will be placed on the page.</div>
</li>
<li class="level2"><div class="li"> Nets ending co-incident with the <strong>pins</strong> of that embedded symbol represent electrical connectivity with the instantiated sub-<strong>circuit</strong> entity.</div>
</li>
</ul>
</li>
</ul>

</div>
<!-- EDIT11 SECTION "Page" [3941-5460] -->
<h1 class="sectionedit12"><a name="brainstorms" id="brainstorms">Brainstorms</a></h1>
<div class="level1">

<p>
(from conversation on MSN/<acronym title="Internet Relay Chat">IRC</acronym> on 10th April 2007 – Peter Brett / Peter Clifton)
</p>
<ul>
<li class="level1"><div class="li"> In order to do back annotation, need to be able to change the board part references for anywhere in the schematic.  It then makes sense to dissociate the concepts of <strong>InstanceID</strong> and <strong>Board Reference</strong>, and use an <strong>override table</strong> that can override an attribute at any given path within the current <strong>circuit</strong> based on a path composed of <strong>InstanceID</strong>s.  <strong>InstanceID</strong>s would be special-cased throughout libgeda as a means for uniquely identifying circuits and instances.  An entry in the override table might have the form ”/id1/id2/id3:refdes:U3”</div>
</li>
</ul>
<ul>
<li class="level1"><div class="li"> It might be useful to allow nets to have attributes, for instance to specify minimum copper width and spacing for a net, independently from the attributes of net segments.</div>
</li>
</ul>
<ul>
<li class="level1"><div class="li"> The schematic editor needs to have sidebars for browsing hierarchy and inspecting attributes.  This needs to include a way of seeing where the attributes have been inherited from.</div>
</li>
</ul>
<ul>
<li class="level1"><div class="li"> We need to do lazy netlisting, on a circuit-by-circuit basis – the netlists should only be combined into a flat netlist when required by a tool (and even then, most tools can potentially make good use of hierarchy information).</div>
</li>
</ul>
<ul>
<li class="level1"><div class="li"> In order to make finding objects by hierarchical path fast (e.g. to implement override tables discussed above) there needs to be a fast way of generating unique identifiers for objects (e.g. 32-bit ints) that can then be used as keys in hashtables.</div>
</li>
</ul>

</div>
<!-- EDIT12 SECTION "Brainstorms" [5461-] --></div>
</body>
</html>
